The present invention relates to a document illumination system and more particularly, to a system incorporating a control mechanism for regulating the light intensity of the illumination source.
In a typical document copier operation, a document lying on a platen is incrementally scanned by an illumination assembly and the reflected image is projected upon a photosensitive surface to form a latent image of the document. The latent image can then be developed and transferred to a copy sheet using well known xerographic techniques. A conventional illumination source is an elongated tubular device such as a fluorescent or tungsten lamp. The efficiency of this type of lamp is usually increased by positioning a reflector in close proximity to the lamp so as to increase the amount of illumination at the scanning slit. U.S. Pat. Nos. 3,926,518 and 4,190,355 disclose representative illumination systems.
A problem inherent in such lamp/reflector systems is the variation in emission of the lamp. Lamp output deteriorates with extended use and the surface can become dirty, impairing emission quality. These emission variations have an adverse affect on image exposure at the photosensitive surface, and, ultimately, of the output copy. Various techniques for compensation for these problems are known to the art. For example, U.S. Pat. Nos. 3,926,518 and 4,017,180 disclose the use of photodetectors on the image side of the projection lens. These photodetectors sample exposure illumination levels and feed back appropriate signals to control the power to the illumination source. A drawback to these types of control systems is that the photodetectors lie along the optical path and interfere, to some extent, with exposure levels.
This problem is met, for some systems, by having the photodetector sense light emissions at some point in proximity to the illumination source but out of the optical path. U.S. Pat. Nos. 4,124,295 and 4,124,294 are representative of two such systems. These systems still have a disadvantage shared by the image side detector systems. Because of the physical limitation (size) of the photodetector, only a small portion of the lamp would normally be viewed. Many lamps, particularly fluorescent types, exhibit variations in the axial illumination profile, (i.e. changing of control points due to mercury migration) and the relatively small area monitored by the detector may not be truly representative of the total or average lamp output.
It is therefore desirable to utilize a photodetector in the direct viewing area of a lamp which will provide an output signal representative of the illumination output along the entire axis of the lamp. According to the invention, this object has been realized by modifying the reflector assembly associated with the lamp so that it performs two functions. The reflector is constructed with a light transmissive interior in optical communication with a photodetector. An axial strip of illumination emitted from the lamp is permitted to enter the reflector interior via a clear axial "window" formed within the reflecting surface. While most of the illumination incident on the reflector is reflected towards the object plane scanning strip, a small portion is sampled and integrated within the reflector interior to provide an average illumination output at the photodetector. The output signal generated by the photodetector can then be compared with a reference signal to maintain the lamp current at a desired value.
The invention then, is related to an illumination system for controlling the illumination of a document in an object plane, the system comprising at least one linear illumination source disposed adjacent to said object plane, an elongated reflector assembly associated with the illumination source and positioned with respect to said source such that at least a portion of the emitted light is directed toward, and is reflected from, the facing portion of said assembly to said object plane, said reflector assembly comprising: a light transmitting interior body, a reflective coating on the surface of said body, said coating having longitudinally extending aperture therethrough along that surface of the assembly or series of apertures or openings directly illuminated by said illumination source whereby a portion of said direct illumination enters into said body, and at least one photosensor in operative association with said body to detect light entering the body through said aperture and propagating along the interior surface, said photosensor adapted to generate output illumination signal for use in controlling said illumination source lens.
In a specific embodiment, the light transmitting aperture on the reflector surface is shaped to purposely weight the signal to compensate for known system profiles. Various preferred materials are also provided which efficiently guide the incident illumination onto the photodetector. A preferred material is a plastic doped with a fluorescent dye. The advantages of this type of material as light collectors have been disclosed in publications such as U.S. Pat. No. 4,251,284 (a solar collector) and in copending U.S. application Ser. No. 257,694 filed on Apr. 27, 1981 now U.S. Pat. No. 4,371,897 issued Feb. 1, 1983, assigned to the same assignee as the present invention.